The body needs vitamin B12 to make blood cells to maintain the health of brain cells and nerves and to help keep the cardiovascular system healthy. Vitamin B12 is a water-soluble vitamin that occurs naturally in animal proteins and is also available in capsule form.

On the Other: Low Potential for Toxicity

Although the body's requirement for vitamin B12 is fairly low (up to 2 micrograms daily), the body absorbs the vitamin poorly, so recommended intakes are usually higher than needed to reflect how the body processes the vitamin. Studies have shown no risk of damage at doses up to 1,000 micrograms per day for five years; doctors prescribe as much as 1,000 to 2,000 micrograms (1 to 2 milligrams) per day of vitamin B12 to patients suffering from deficiency.

Bottom Line

A maximum upper limit of vitamin B12 has not been established in the U.S. because of its low potential for harm. The recommended daily intake for vitamin B12 is 2.4 micrograms per day, but vegetarians and patients with vitamin deficiencies, certain types of anemia or symptoms of Alzheimer's disease frequently take much larger daily doses to ensure adequate nutritional needs.

Source:

NIH: Vitamin B12 Fact Sheet

NIH Medical Encyclopedia: About Vitamin B12